1. Field of the Invention
The present invention relates to a power amplifier circuit which amplifies an inputted input signal and outputs it as an output signal.
2. Description of the Related Art
FIG. 11 is a diagram showing the configuration of a related power amplifier circuit 5 using bipolar transistors and its peripheral circuit, and FIG. 12 is a diagram showing the static characteristic of an amplifying part transistor Q in FIG. 11.
As shown in FIG. 11, a voltage of 3.6 V, for example, is supplied from a power supply 10 to a DC/DC converter 20. The DC/DC converter 20 converts the supplied voltage into, for example, 1.5 V and supplies it to a collector of the amplifying part transistor Q in the power amplifier circuit 5. The amplifying part transistor Q is composed of an NPN bipolar transistor. The voltage which is supplied by the DC/DC converter 20 is variable, and in some cases a high voltage (3.6 V, for example) is supplied, while in other cases a low voltage (1.5 V, for example) is supplied.
A high-frequency signal current Isg is supplied to a base of the amplifying part transistor Q from a signal generating circuit 30 via a capacitive element C1, and a bias current Ibias from a bias circuit 40 is also supplied thereto. Namely, a base current Ib=Ibias+Isg is supplied to the base. An emitter of the amplifying part transistor Q is grounded.
The bias circuit 40 includes NPN bipolar transistors Q1 to Q4 and a resistance R1. The bipolar transistors Q1 and Q2 and the resistance R1 constitute a switching control circuit 50, and the bipolar transistors Q3 and Q4 constitute a bias current generating circuit 60.
As concerns a control voltage Vcon to be supplied to the switching control circuit 50, a voltage (3.6 V, for example) is supplied when the power amplifier circuit 5 is on, and no voltage (namely, 0 V) is supplied when it is off. In the bias current generating circuit 60, the bipolar transistor Q3 is turned on/off according to the on/off of the control voltage Vcon, and when it is on, the bias current Ibias is supplied from a reference voltage Vref to the base of the amplifying part transistor Q, and when it is off, the bias current Ibias is not supplied.
The switching control circuit 50 changes the amount of the bias current Ibias flowing through the bipolar transistor Q3 by changing the amount of a control current Icon to be supplied to a base of the bipolar transistor Q3 according to its ambient temperature (ambient temperature of the bipolar transistors Q1 and Q2). Hence, the switching control circuit 50 prevents thermal runaway of the power amplifier circuit. In addition, the bipolar transistors Q1 and Q2 also monitor the ambient temperature of the amplifying part transistor Q.
An output node N0 of this power amplifier circuit is provided on the collector side of amplifying part transistor Q and outputs a voltage output OUT. A load impedance ZL is connected to the output node N0.
As shown in FIG. 12, the amount of a current Ic flowing through the amplifying part transistor Q is determined by the base current Ib (Ib1 less than Ib2 less than Ib3 less than Ib4 less than Ib5 less than Ib6) which flows into the base of the amplifying part transistor Q. As stated above, the base current Ib is the sum of the bias current Ibias from the bias circuit 40 and the signal current Isg from the signal generating circuit 30. The bias current Ibias is constant unless temperature changes, and hence the voltage of the voltage output OUT is determined by the high-frequency signal current Isg from the signal generating circuit 30.
Incidentally, as shown in FIG. 12, from the characteristics of the bipolar transistor, an amplitude of the voltage output OUT can be widely at high output levels at which the voltage Vc is high, while the amplitude of the voltage output OUT can be only narrowly at low out levels at which the voltage Vc is low.
However, as can be seen from FIG. 11 and FIG. 12, even if the voltage Vc is reduced at low output levels, the amount of the current Ic flowing through the amplifying part transistor Q is unchanged. Since power consumption (DC loss) is determined by voltage Vcxc3x97current Ic, power consumption is reduced only through a fall in the voltage Vc.
When the voltage Vc is at low output levels, however, it does not matter if the amplitude of the voltage output OUT is small, and hence if it is possible to decrease the amount of the current Ic and thereby reduce power consumption, it is more desirable.
In order to accomplish the aforementioned and other objects, according to one aspect of the present invention, a power amplifier circuit, comprises:
an amplifying part transistor including a bipolar transistor to which a first supply voltage is supplied as a driving voltage and which amplifies an input signal inputted to a base of the amplifying part transistor so as to output the input signal;
a bias current generating circuit which generates a bias current for biasing the base of the amplifying part transistor and supplies the bias current to the base of the amplifying part transistor when the amplifying part transistor performs an amplification operation; and
an additional bias current generating circuit which supplies an additional bias current to the base of the amplifying part transistor in addition to the bias current according to the first supply voltage.